Both the LCD monitor and LCD TV apparatus use the cold cathode fluorescent lamp as a backlight because this lamp has the best illumination efficiency. Therefore, large-size LCD panels usually contain multiple cold cathode fluorescent lamps. There are various methods of implementing a DC/AC inverter to drive the multiple cold cathode fluorescent lamps. Topologies such as half-bridge, full-bridge, and push-pull circuits are examples.
FIG. 1 shows a schematic drawing of a conventional center-tap primary windings push-pull topology used to drive cold cathode fluorescent lamps in accordance with the prior art. DC power 110 provides DC power to the push-pull circuit. DC power 110 is connected to the primary windings 810–814 of the transformers 310–314 which are connected in parallel. Each secondary winding 710–714 of the transformers 310–314 is coupled to a cold cathode fluorescent lamp circuit 410–414. There is a center-tap on each primary winding 810–814. Two power switches 212 and 214 are coupled to each primary winding 810–814. However, conventional push-pull circuit is limited due to undesired voltage spikes which are caused by the leakage-inductance energy of the transformer at the power switches when they are turned off. In addition, for a high-voltage application, the transformer primary winding needs more turns than those in battery-input applications, which will increase the size and the cost of the transformer. And the transformer winding ratio of FIG. 1 is:Ratio≈VOUTrms/VIN  (1)Where VOUTrms is the maximum output voltage and VIN is the minimum input voltage of the transformer. The voltage spike across the switch is usually suppressed by a snubbed circuit to absorb the leakage energy. This passive implementation reduces the power conversion efficiency and increases the system cost with additional parts.
FIG. 2 shows another schematic drawing of a conventional half-bridge topology used to drive cold cathode fluorescent lamps in accordance with the prior art. DC power 110 provides DC power to the half-bridge circuit. Two switches 212 and 214 are coupled to each primary winding 810–814 of the transformers 310–314. Each secondary winding 710–714 of the transformers 310–314 is coupled to a cold cathode fluorescent lamp circuit 410–414. However, for the bridge-type (either half-bridge or full-bridge) circuitry, a level shifter circuit and a high-side driver is needed for the power transistor connected to the input voltage source. For high-voltage applications, this will increase the circuit cost significantly and suffers reliability of switching high-voltage signals.